This invention relates to down-hole devices for imparting rotary motion, and is more particularly concerned with devices for converting the kinetic energy of the mud flow passing along a drill string into rotary motion for driving a generator or a measuring or signalling instrument (hereinafter referred to as devices "of the type defined").
Because of space limitations down-hole for the accommodation of batteries, and so as to avoid the problems associated with the need for frequent battery replacement, there are many circumstances in which it is desirable for electrical power to be generated down-hole by a turbogenerator driven by the mud flow or for mechanical energy required for a moving part down-hole to be generated directly by an impeller driven by the mud flow. Devices for performing these functions down-hole are known. For example, the applicant's British Patent Specifications Nos. 2,082,653A and 2,087,951A disclose an arrangement in which a mud-driven impeller drives both an electrical generator and a linearly-displaceable throttling member for generating pressure pulses in the mud flow with the object of transmitting measurement data to the surface.
However, such known devices require the use of impeller bearings which are capable of accommodating a substantial thrust due to the mud flow acting on the impeller, and it is difficult to produce suitable bearings which have a long service life. It is particularly important that devices which operate down-hole should not require frequent repair or replacement as such repair or replacement can be very costly in terms of lost drilling time. Furthermore it can be difficult to ensure a long service life for a down-hole device in view of the hostile conditions down-hole and the abrasive nature of the mud.
It is an object of the invention to provide a device of the type defined which is capable of a long service life down-hole.